EP-A-2 677 176 discloses a rotary machine of the electric compressor type, comprising a stator assembly, a rotor assembly, magnetic bearings and auxiliary bearings. The rotary machine comprises sensors for monitoring the position of a central shaft belonging to the rotor assembly and to detect any deviation from nominal position.
In a known manner, the auxiliary bearing equipping a rotary machine may be an anti-friction ball bearing, having an outer ring held in the stator assembly and an inner ring separated by a radial gap from the rotor assembly. The auxiliary bearing is directly exposed to its environment, preferably with a dry and permanent lubrication, without permanent greasing.
In case of failure of the magnetic bearing, the rotary machine is protected by the auxiliary bearing. In normal operation, when the magnetic bearing is working properly, the inner ring of the auxiliary bearing is not rotating and there is no contact between the rotor and the auxiliary bearing. Nevertheless, aerodynamic friction may drag the inner ring into rotation, which will on long term remove the permanent lubrication then damage the auxiliary bearing. Normally, the “break-away torque” needed to start the inner ring rotation of a new auxiliary bearing is higher than the aerodynamic effect. However improper operation or malfunction can sometimes result in a highly reduced break-away torque. Undetected rotation of the auxiliary bearing leads to its deterioration, so that the auxiliary bearing does not protect the rotor assembly anymore from rubbing the stator assembly in case of magnetic bearing failure or process overload.